1. Related Application
This application claims a priority of a previously filed TW Patent Application No. 101142570 filed on Nov. 14, 2012, entitled as “Lighting apparatuses and driving methods regarding to light-emitting diodes”, and the entire contents of which are hereby incorporated by reference herein in its entirety.
2. Technical Field
The present disclosure relates to a light-emitting diode and in particular to a lighting apparatus having a high color rendering index (CRI) using a light-emitting diode and related driving methods.
3. Description of the Related Art
The light-emitting diode (LED) is a lighting source made of semiconductors. Since the LEDs have the characteristics of long operational life, low power consumption, and light weight, the LEDs are gradually replacing traditional lighting parts such as incandescent light bulbs. The colors of lights emitted by the LEDs are determined by the semiconductor material used to manufacture the LEDs.
Some semiconductor materials of LEDs are alloys of groups III-V, such as gallium nitride (GaN). Generally, the alloys are epitaxially deposited by layer on a substrate made of silicon carbide or sapphire base during the manufacturing process. The alloys can be doped with p-type or n-type impurities to adjust the electrical properties. The LED based on GaN emits a light having a color close to ultraviolet (UV) or blue in a light spectrum.
The phosphor layer is coated on LED for lighting application. The phosphor is a photoluminescence material which absorbs an electro-magnetic wave on a part of the light spectrum and emits another electro-magnetic wave on another part of the light spectrum. Thus, when LED is covered by phosphor layer, the light emitted by the LED which is not absorbed by the phosphor layer and a light emitted by the phosphor layer are mixed to be a light having a desired color and a luminance.
Take white light as an example, the LED emitting white light incorporates a blue light LED which has a light-emitting layer made of InGaN covered by a phosphor layer which turns a part of the blue light emitted from the blue light LED into a yellow light or yellow-green light. When the white light LED is turned on, the semiconductor in the blue light LED turns the electrical power into a blue light (or UV light) and part of it is absorbed by the phosphor and becomes a yellow-green light (or yellow light). Since the yellow light or yellow-green light is substantially the complementary color of the blue light, the combination of the yellow light and the blue light is considered as a white light in human's eye. Some characteristics of a white light source are important to be taken into consideration, such as correlated color temperature (CCT) and color rendering index (CRI), to illustrate how close the light emitted by the white light source is when compared with a light in nature.
Color temperature is indicated by Kelvin scale (° K) which represents a color of an ideal black body at that color temperature. Generally speaking, an incandescent light bulb is close to an ideal black body on a light spectrum. While an incandescent light bulb is heated to 2000° K, a red light is emitted. With an increase of the temperature, the light turns from orange-red to yellow gradually. When it is 5000° K, the light is substantially a white light. When it is 8000° K, the light is substantially a blue light. Simply speaking, the higher the temperature is the larger blue proportion the light has; the lower the temperature is the larger red proportion of the light has. Because many light spectrums of artificial light sources are not the same as the light spectrum of the ideal black body, the correlated color temperatures (CCT) are used to represent their color temperature. If an artificial light source emits a light having a combination of color which is similar to a combination of a light emitted by an ideal black body at a color temperature, the CCT of the artificial light source is defined as the color temperature of the light emitted by the artificial light source. Many methods are used to estimate a CCT of a white light from its chromaticity coordinates. Although the methods are different, the difference of the estimated CCTs is small.
Generally, a definition of CRI is the performance of eight chosen colors radiated by a white light source. A highest CRI value of a white light source is 100 when the eight colors are fully reproduced. The lower CRI indicates a larger bias is appeared when the eight colors are radiated by that white light source. There is other CRI definition using fourteen colors for evaluation. In this specification, the CRI generally refers to the definitions described above or any results from similar definition.
A well-known white light LED adopts a combination of a blue light diode having a peak-emission wavelength about 440 nm to 480 nm and a YAG phosphor consisted of cerium doped yttrium aluminum. The YAG phosphor turns a part of the blue light into yellow-green light. A light combination of a blue light and a yellow-green light is considered as a white light to human's eye.
Recently, the white light LED is difficult to reach a desired CRI and a CCT. Since the light emitted by the white light LED usually has two peaks on a light spectrum locate near blue light and yellow-green light respectively and lacks a red light having a lower wavelength needed to adjust CRI and CCT. Therefore, it is difficult to lower the CCT of a white light LED mentioned above to below 5000° K, and the CRI is normally lower than 75.
A method to decrease CCT and to increase CRI of a white light LED is to change the composition of the phosphor. For example, the phosphor layer has at least two phosphors: one is a YAG phosphor which is used to generate yellow-green light, and another phosphor is used to generate a red light. Then, the white light LED has peaks on frequencies of a spectrum near blue light, yellow-green light, and red light respectively. A desired CCT and a CRI are achieved by adjusting the ratios and peak values of the peaks. But, the energy transition efficiency of the phosphor which generates a red light is poor and the light emitting efficiency of a white light LED is reduced.